1. Field of the Invention
The present invention relates to a television tuner for receiving a television signal in the UHF band or the VHF band.
2. Description of the Related Art
FIG. 5 shows the configuration of a conventional television tuner. An input terminal 31, to which a television signal in the VHF band or the UHF band is input, is coupled to a VHF tuning circuit 41 of a VHF tuner unit 40. In the subsequent stage of the VHF tuning circuit 41, a first FET (dual-gate FET) 42 which functions as a high-frequency amplifier is provided.
The input terminal 31 is also coupled to a UHF tuning circuit 51 of a UHF tuner unit 50 via a first switching diode 33. In the subsequent stage of the UHF tuning circuit 51, a second FET 52 which functions as a high-frequency amplifier is provided.
The VHF tuner unit 41 includes an inductor 41b, one end thereof being grounded for high frequencies via a DC cutting capacitor 41a; three inductors 41c, 41d, and 41e, sequentially connected in series to the other end of the inductor 41b for high frequencies; a first varactor diode 41f, the cathode thereof being connected to the inductor 41e on the hot-side end and the anode thereof being grounded, thus being connected in parallel to the four inductors 41b, 41c, 41d, and 41e; and a second switching diode 41g connected in parallel to the middle inductors 41c and 41d for high frequencies.
The node between the two middle inductors 41c and 41d is coupled to a filter 32. The anode of the second switching diode 41g is connected to the anode of the first switching diode 33 for DC voltage.
To the node between the DC cutting capacitor 41a and the inductor 41b, a predetermined bias voltage which is obtained by dividing a power supply voltage B by resistors is applied. The bias voltage is applied to each of the anodes of the first and second switching diodes 33 and 41g via the inductors 41b, 41c, etc.
The cathode of the second switching diode 41g is connected to a first switching terminal 34a of a band switching circuit 34 via a resistor 41h. 
The node between the inductor 41e on the hot-side end and the cathode of the varactor diode 41f is coupled to a first gate of the FET 42, which functions as an input terminal of the FET 42. The first gate is connected to the collector of the switching transistor 35. A power supply voltage B is applied to the collector of the switching transistor 35 via a power supply resistor 36, and the emitter of the switching transistor 35 is grounded.
The cathode of the first switching diode 33 is connected to the collector of the switching transistor 35 via a resistor 37.
Furthermore, the cathode of the first switching diode 33 is coupled to the node of the inductors 51a and 51b constituting the UHF tuning circuit 51 of the UHF tuner unit 50. A varactor diode 51c is connected in parallel to the two inductors 51a and 51b. The UHF tuning circuit 51 is coupled to a first gate of the second FET 52, which functions as an input terminal of the second FET 52.
To the first gate of the second FET 52, one end of a peaking coil 52a is connected. The other end of the peaking coil 52a is grounded for high frequencies, connected to the base of the switching transistor 35, and connected to a second switching terminal 34b of the band switching circuit 34.
Although not shown, tuning voltages for changing the tuning frequencies of the tuning circuits 41 and 51 are applied to the cathodes of the first and second varactor diodes 41f and 51c, and AGC voltages for controlling gains are applied to second gates of the first and the second FETs 42 and 52, respectively.
Furthermore, a mixer, etc. for frequency conversion is provided in the subsequent stage of each of the FETs 42 and 52, although not shown and description thereof omitted.
The band switching circuit 34 generates switching voltages for switching the operation status of the VHF tuner unit 40 and the UHF tuner unit 50 and for switching the reception band of the VHF tuner unit 40. More specifically, the voltage applied to the first switching terminal 34a goes to high level (e.g., the power supply voltage B) when a television signal in the VHF low band or a television signal in the UHF band is to be received, and the voltage goes to low level (e.g., 0 volts) when a television signal in the VHF high band is received. The voltage applied to the second switching terminal 34b goes to high level only when a television signal in the UHF band is to be received, and the voltage goes to low level when a television signal in the VHF band (both the low band and the high band) is to be received.
In the configuration described above, when a television signal in the VHF band is to be received, the second switching terminal 34b goes to low level, deactivating the second FET 52. Thus, the switching transistor 35 is turned off, whereby a bias voltage is applied to the first gate of the first FET 42 via the power supply resistor 36. The bias voltage is appropriately arranged to be on the order of 1.7 volts by the internal circuit of the first FET 42. Furthermore, the first switching circuit 33 is turned off, so that the television signal is inhibited from entering the UHF tuning circuit 51.
When a television signal in the VHF low band is to be received, the first switching terminal 34a goes to high level, turning off the second switching diode 41g, whereby the VHF tuning circuit 41 tunes to the low band.
When a television signal in the VHF high band is to be received, the first switching terminal 34a goes to low level, turning on the second switching diode 41g. Thus, the inductors 41c and 41d become effectively absent in the VHF tuning circuit 41, whereby the VHF tuning circuit 41 tunes to the high band.
When a television signal in the UHF band is to be received, the second switching terminal 34b goes to high level, whereby a bias voltage is applied to the first gate of the second FET 52, activating the second FET 52. Furthermore, the switching transistor 35 is turned on, whereby the voltage applied to the first gate of the first FET 42 becomes substantially 0 volts, deactivating the first FET 42. Furthermore, the first switching diode 33 is turned on in response to the switching transistor 35 being tuned on, so that the television signal in the UHF band is input to the UHF tuning circuit 51 of the UHF tuner unit 50.
In the above configuration, the output capacitance (the internal capacitance between the collector and the emitter) of the switching transistor 35 is applied to the first gate of the first FET 42 when the switching transistor 35 is turned off and the first FET 42 is activated. As a result, the tuning frequency range of the VHF tuning circuit 41 becomes narrower, and in particular, tuning to the high hand becomes difficult, causing deterioration of NF.
Furthermore, when the switching transistor 35 is turned off, the first switching diode 33 is also tuned off; thus, the bias voltage applied to the anode of the first switching diode 33 must be lower than 1.7 volts (the bias voltage applied to the first gate of the first FET 42). If the voltage is arranged, for example, to be on the order of 1.0 volt, the reverse voltage between the anode and the cathode of the first switching diode 33 is lowered, the first switching diode 33 thus causing a distortion when a television signal of a large magnitude is input.
A countermeasure against the above problem would be to lower the bias voltage applied to the first switching diode 33. However, in accordance with the approach, the resistance of the resistor 41h on the cathode side must be small in order that a sufficient current flow through the second switching diode 41g when a high band signal is to be received. The resistor 41h, which is connected in parallel to the inductor 41b, lowers the Q of the VHF tuning circuit 41 and increases insertion loss, deteriorating NF in the high band.